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Having reviewed the characteristics of the classical DC machines in the last section, this section will give the same treatment to the classical AC motors. The discussion begins with the AC series motor. As previously mentioned, the ordinary DC series motor can often give a satisfactory account of itself when operated from the AC power line. However, much better efficiency and commutation are obtained when a series motor operated from an AC source is designed for AC use. It then performs well on DC also. Such a machine is known as a universal motor. Because of economic factors, not all universal motors make use of all the design and structural techniques that are known to optimize AC operation. This is particularly true for small fractional-horse power sizes and reflects the general trend for series motors under ¼ horsepower.
Operation of the series motor from AC power
Compared with DC series motors, the AC type can differ in at least some of the following respects:
• The AC motor tends to have fewer series-field ampere turns but tends to have a higher number of ampere turns on the armature.
• The field structure of the AC motor is highly laminated to reduce eddy current loss.
• In larger sizes (greater than ¼ horsepower), the ac-type series motor uses distributed, rather than concentrated, poles.
• In the larger machines, the ac-type series motor is likely to have a greater number of poles.
• When compensating windings are used, they are often inductively, rather than conductively, associated with the armature. That is, these windings are short circuited and are not physically connected in the motor circuit.
• In the larger sizes, resistance is sometimes introduced between the armature conductors and the commutator segments. This is a commutation aid, and protects the brushes from high short-circuit currents during starting.
• In the AC series motor, there is more emphasis on making the air gap small.
Most series motors of less than 1/4-horsepower rating are classified as universal motors if their design permits satisfactory operation on AC up to 60 Hz. Motors with larger ratings are often referred to simply as AC series motors. Unless specially designed, most small fractional-horsepower universal motors won’t develop the same speed and torque characteristics on both AC and dc. For a large number of applications, however, the behavior will be sufficiently similar to satisfy requirements. If operation is desired at frequencies above 60 Hz, custom designing is usually involved.
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